1. Field of the Invention
The present invention is directed to an x-ray diagnostic apparatus for tomosynthesis or laminographic imaging with a patient positioning table, with an x-ray source which emits an x-ray beam on one side of the patient positioning table, an x-ray detector arranged on the other side of the patient positioning table for capturing the x-ray beam and for producing digital images, a device for moving the x-ray source, a device for adjusting the x-ray detector, and a device for superimposing the digital images so that only the details of an examination subject disposed in a specific longitudinal slice are sharply imaged.
2. Description of the Prior Art
An x-ray diagnostic apparatus of this type is known, for example, from German OS 27 12 320, corresponding to U.S. Pat. No. 4,149,082 wherein images of a specific slice of a patient lying on a patient positioning table are produced as a result of the movement of the x-ray beam. Only those points are thereby sharply imaged that lie in the selected slice plane. Points in the body of the patient that lie above or below the selected slice plane are automatically precluded from contributing to the image, due to the movement of the x-ray beam, so that they are not perceptible.
A digital solid-state flat detector with radiation-sensitive elements formed by amorphous silicon arranged in a matrix configuration is disclosed, with an indirect x-ray converter, such as, for example, a scintillator composed of caesium iodide (Csl) has recently been developed for radiography and fluoroscopy. Digital solid-state flat detectors having a direct x-ray converter, such as e.g. selenium, are also known. Such flat detectors generate an image signal in digital form, corresponding to the resolution produced by the number of elements. For this reason, they are well-suited for the digital further processing of the image signals and in particular, also for the use of reconstructive methods. Tomosynthesis, laminographic imaging, or classical tomography each represent such a reconstructive imaging method, that is also called digital tomosynthesis given digital processing.
Due to the production process and the complex read-out electronics, these new digital detectors customarily exhibit faults that can be amplified in the processing of the signals. Such faults can cause individual, dropped out pixels or pixel groups as well as vertical and horizontal stripe patterns, referred to as offset faults and readout electronic stripes.
These artifacts are particularly disruptive given tomosynthesis, wherein the individual projections are conventionally acquired with very low dosage and the radiography detector used, among others, is specified for higher dosage values with respect to the faults. In digital tomosynthesis, individual exposures of a subject are made from different viewing directions that are subsequently shifted and summed. To improve image quality, the exposures are often digitally filtered before or after the superimposition using filters that usually exhibit a high pass characteristic and operate in the scanning direction. Since n individual projections are added for the preparation of the overall image, the x-ray dosage of an individual projection approximates an nth of that of the overall image. Customarily, n lies in the range between 10 and 100 so that a very low dosage value results for an individual projection. Given such limited dosage values, however, the described faults become particularly conspicuous. The fault specifications for the detectors are mostly oriented to the customary, radiographic exposure requirements, i.e. at essentially higher dosage values than are required for the tomosynthetic individual projections.
Given the flat detectors used heretofore for projection radiography, either a preprocessing is implemented using appropriate calibration and correction software, or the faults are limited such that they can be tolerated given the radiographic and fluoroscopic dosage values used. The requirements for the quality of the detectors with respect to the artifacts is then higher, the lower the prescribed dosage is. No routine application using grossly planar, solid-state flat detectors is known for such low dosage values as are required for the tomosynthetic individual projections.
An x-ray detector arrangement for CT-scanning systems is known from German OS 195 25 605, wherein the individual detector elements have a parallelogram-type input surface. The goal of this type of structure is to reduce the effective distance of the individual detector elements, whereby the topical resolution can be increased. The detector elements are arranged in a parallelogram grid that has, as a result, a specific structurally determined grid arrangement of the elements.
An object of the present invention is to provide an x-ray diagnostic apparatus of the type described above that, without pre-processing, is insensitive to artifacts arising from detector errors.
The object is inventively achieved in an apparatus wherein the x-ray detector is rotated relative to the tomosynthesis direction or the slice direction by an angle xcex1. As a result of the rotation of the flat detector at an angle a within the detector plane relative to the tomosynthesis direction or slice direction of the x-axis, vertical and horizontal stripe shaped detector faults are not noticeable, for example, or are at least strongly suppressed since they are approximately averaged out. A two-dimensional detector of conventional structure having a rectangular intrinsic scanning grid is rotated at an angle against the scanning direction.
It has proven to be advantageous for the x-ray detector to be a digital large area solid-state flat detector.
In accordance with the invention, the angle a can lie in the range of 2xc2x0 to 88xc2x0, particularly 5xc2x0 to 20xc2x0, preferably 10xc2x0.
It is advantageous for the angle a of the flat detector to be adjustable depending on the operating mode, artifact structure, readout means of the specifically observed detector and tomosynthesis movement so that it can be optimally adapted to each situation. The angle xcex1 of the flat detector is variably adjustable from individual projection to individual projection. As a result, it becomes possible to use the detector non-rotated in the radiography operation. In the tomosynthesis mode, the angle a can be optimized according to the artifact structure, readout direction of the specifically observed detector and tomosynthesis movement. Also, an angle that is variable from individual projection to individual projection can be attained.
Besides a normal x-ray diagnostic apparatus, a C-arm device can also be used according to the invention.
It has proven to be advantageous for the x-ray source to be moved in a first direction and the x-ray detector is oppositely moved in a second direction opposite that of the first. Alternatively, the x-ray source and the x-ray detector move isodirectionally in the same direction.